1. Field of the Invention
The present invention relates to lithium rechargeable batteries. More particularly, the present invention relates to lithium rechargeable batteries having excellent safety, improved short circuit resistance and improved heat resistance.
2. Description of Related Art
Recently, portable electronic instruments have been designed to have low weight and compact size. As such, batteries used as driving sources for such instruments have been required to have low weight and high capacity. Active and intensive research and development have been conducted into lithium rechargeable batteries. Lithium rechargeable batteries typically have drive voltages of 3.6 V or higher, which is at least three times higher than the drive voltages of Ni—Cd batteries or Ni-MH batteries which are currently widely used as power sources for portable electronic instruments. Moreover, lithium rechargeable batteries provide higher energy densities per unit weight than do Ni—CD or Ni-MH batteries.
A lithium rechargeable battery generates electric energy through redox reactions occurring during lithium ion intercalabon/deintercalation in the cathode and anode. A lithium rechargeable battery is manufactured by placing an organic electrolyte or a polymer electrolyte between a cathode and an anode, each of which includes a material capable of reversible lithium ion intercalation/deintercalation as active material.
A typical lithium rechargeable battery includes an electrode assembly formed by winding an anode plate, a cathode plate and a separator positioned between the electrode plates into a predetermined shape such as a jelly-roll shape. The battery further includes a can for housing the electrode assembly, an electrolyte, and a cap assembly mounted to the top of the can. The cathode plate of the electrode assembly is electrically connected to the cap assembly via a cathode lead, and the anode plate of the electrode assembly is electrically connected to the can via an anode lead.
The separator in a lithium rechargeable battery functions basically to separate the cathode and the anode from each other to prevent short circuits. Add itonally, the separator maintains high ion conductivity and allows infiltration of the electrolyte necessary to carry out electrochemical reactions in the battery. Particularly, in lithium rechargeable batteries, separators must also prevent movement within the battery of substances capable of inhibiting such electrochemical reactions. The separator may also function to ensure the safety of the battery under abnormal conditions.
Generally, the separator includes a polyolefin based microporous polymer membrane (such as polypropylene or polyethylene), or a multilayer membrane including multiple sheets of such membranes. Such conventional separators consist of sheet-like or film-like porous membrane layers, and are disadvantageous in that if heat emission occurs due to an internal short circuit or overcharge, the pores of the porous membrane may become blocked and the sheet-like separator may shrink. If the sheet-like separator shrinks due to such internal heat emission of the battery, the area covered by the separator may decrease and the cathode and anode may directly contact each other, resulting in ignition and explosion of the battery.
To ensure the safety of batteries upon heat emission caused by short circuits, these film-like separators often have so-called shutdown actions that interrupt lithium ion movement (i.e. current flow) by blocking the pores of the separator with a softened polypropylene or polyethylene resin. However, these separators are still disadvantageous when an internal short circuit occurs. For example, using a nail test (perforation) to simulate an internal short circuit condition, it can be shown that the heat emission temperature may locally reach several hundred degrees C depending on the test conditions. Consequently, the porous membrane layer is deformed by the softening or loss of the resin. Further, in the nail test, the test nail perforates the cathode and the anode, thereby causing an abnormal overheating phenomenon. Therefore, a separator membrane using the aforementioned shutdown action with a softened resin cannot provide an absolute safety measure against internal short circuits.
Additionally, lithium dendrites may be formed on a film-like separator upon overcharge of the lithium rechargeable battery. This occurs because there is typically a gap between the anode and the film-like separator. Lithium ions that cannot infiltrate the anode accumulate in the gap between the anode and the film, resulting in the precipitation of lithium metal. If lithium precipitation occurs over the entire surface of the film, such lithium dendrites may penetrate through the film-like separator so that the cathode comes into direct contact with the anode. At the same time, side reactions may occur between lithium metal and the electrolyte to cause heat emission and gas generation, resulting in the ignition and explosion of the battery.
Moreover, a film-like separator cannot function as a separator separating the cathode and anode when it is not in an aligned state. The separator may fall out of an aligned state due to vibration or dropping. When this occurs, the cathode and anode come into direct contact with each other, thereby generating a short circuit and resulting in battery malfunction. In addition, a film-like separator may be wound incorrectly during battery manufacture, resulting in an increase in the production of defective products and a decrease in safety. Further, a film-like separator cannot be used at high temperatures of 100° C. or greater because the film melts at such high temperatures.